Generated XER Decode Methods

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The code generated to decode XML messages is different than that of the other encoding rules. This is because off-the-shelf XML parser software is used to parse the XML documents to be decoded. This software contains a common interface known as the Simple API for XML (or SAX) that is a de-facto standard that is supported by most parsers. ASN1C generates an implementation of the content handler interface defined by this standard. This implementation receives the parsed XML data and uses it to populate the structures generated by the compiler.

The default XML parser used for C# is the MSXML parser that is part of the Microsoft .NET framework. As mentioned, since SAX is a de-facto standard, it should be a relatively straightforward process to use the generated handlers with any other parser.

A diagram showing the components used in the XML decode process is as follows:

Step 1: Generate code:

Step 2: Build Application:

ASN1C generates code to implement the following methods defined in the SAX content handler interface:

	StartElement
 

 
	Characters
 

 
	EndElement
 

The interface defines other methods that can be implemented as well, but these are sufficient to decode XER encoded data. These methods are added to an inner SAX handler class generated for each ASN.1 production.

The procedure to invoke the generated decode method is as follows:

• Instantiate an XmlSaxParser object.
  
• Instantiate a generated C# <ProdName> object to hold the decoded message data.

• Invoke the <ProdName> object decode method passing the reader created in step 1 and the URI of the XML document to be parsed. This method initiates and invokes the XML parser's parse method to parse the document. This, in turn, invokes the generated SAX handler methods.
  
• Methods within the <ProdName> object can now be used to access the decoded data. The member variables that were declared to be public can be accessed directly.

• Error handling is accomplished using a try-catch block to catch SAX exceptions.

A program fragment that could be used to decode an employee record is as follows:

public class Reader { public static void main (string args[]) { string filename = "employee.xml"; try { // Create an XML reader object XmlSaxParser reader = XmlSaxParser.NewInstance(); // Read and decode the message PersonnelRecord personnelRecord = new PersonnelRecord (); personnelRecord.Decode (reader, filename); if (trace) { System.Console.Out.WriteLine ("Decode was successful"); personnelRecord.Print ("personnelRecord"); } } catch (Exception e) { System.Console.Out.WriteLine (e.Message); Asn1Util.WriteStackTrace(e, Console.Error); return; } } }

Figure 9 - XER Decoding Example